In motor vehicles, it is known that the servo brake system and the servo steering system mostly draw their auxiliary energy from an underpressure store which is connected to an intake manifold. The intake manifold serves to supply the engine with the air (that is, the necessary oxygen) for the combustion. Here, the intake manifold underpressure is stored in the underpressure store which is connected via a check valve to the intake manifold. For an adequate boosting of the braking force, there must have, in each case, an underpressure been present for an adequately long time in the intake manifold in order to ensure the corresponding underpressure in the underpressure store. When there is low pressure in the intake manifold, air flows from the store into the intake manifold. The pressure in the underpressure store then drops to the pressure in the intake manifold. When the brake is actuated, the underpressure store is connected via a valve to a control element which boosts the braking force. Air then flows into the underpressure store and thereby increases the store pressure.
In internal combustion engines of the above type, a throttle flap is provided in the intake manifold with which the air supplied to the combustion chamber can be adjusted. In conventional internal combustion engines and especially in the spark-ignition engine, the throttle flap closes when the driver takes the foot off the accelerator pedal during braking whereby the store underpressure is generated and maintained. For these internal combustion engines, it is especially ensured that the underpressure store can provide the underpressure, which is required for the servo brakes, even during longer braking operations.
In newer internal combustion engines equipped with gasoline direct injection (GDI), this is, however, not always ensured. For example, the throttle flap is opened so wide during a heating operation of a catalytic converter that there is no longer an adequate underpressure present in the intake manifold and therefore the underpressure required for the servo system(s) can no longer be made available also in the underpressure store.
Intake manifold underpressure for servo functions is available only to a limited extent in internal combustion engines having GDI wherein the throttle flap is controlled independently of the position of the accelerator pedal transducer. Examples for this are operating states during intake manifold injection with a retarded ignition angle for heating the catalytic converter during warmup of the engine. Here, a desired deterioration of efficiency must be compensated by opening the throttle flap. This leads to an increase of the intake manifold pressure. The known stratified-charge operation in the case of gasoline direct injection is a comparable operating state wherein the throttle flap is fully opened even at low load and therefore no intake manifold pressure is available.
However, especially in the operation of vehicles at high elevations such as during travel in mountainous country, the difference with respect to the ambient pressure is no longer sufficient for the servo functions.
The servo braking system is especially critical with respect to safety in vehicles. If no adequate underpressure is available, no braking force is available or there can be no operation of the engine in the stratified-charge mode because of safety reasons and this leads to a deterioration in the exhaust gas or consumption.
It is therefore already known to solve the problem in that the throttle flap opening is so designed that there is always sufficient underpressure. This has the consequence (for example, during heating of the catalytic converter) that the design of the throttle flap opening is not optimal with respect to exhaust gas. For GDI, an underpressure switch is, inter alia, utilized. If the pressure in the brake booster rises above a threshold value, then there is a switchover from stratified operation to homogeneous operation.
As an alternate solution, an additional underpressure pump or servo pump can be built into the vehicle in order to compensate for the difference pressure which is perhaps not present. This, however, leads to increased cost.
In motor vehicles which are equipped with GDI-operated spark injection engines, the following problem is especially presented. The underpressure for the brake booster is made available in conventional spark-ignition engines with intake manifold injection via the intake manifold underpressure. In GDI engines, this intake manifold underpressure is not available when the engine is operated in the stratified-charge mode of operation.
However, it must be ensured at all times that sufficient underpressure is available for the brake booster. It is, however, unnecessary to continuously supply the brake booster with underpressure when there is an underpressure store available; instead, it is sufficient to provide a regular "fill-up" of the underpressure store with underpressure.
In known internal combustion engines, the above-mentioned problematic is solved by the use of a pressure switch or a pressure sensor. The purpose of the switch is to transmit an appropriate signal to an engine control apparatus when the underpressure in the brake booster is too low. The engine control apparatus then requests a switchover from the stratified-charge mode of operation to the homogeneous mode of operation and the required intake manifold underpressure is thereby again made available. These internal combustion engines have the disadvantage that additional costs are incurred for the mentioned switches or sensors.
Further disadvantages of the known internal combustion engines result from the frequency with which there is a switchback to the homogeneous mode of operation when the pressure sensor responds. The fuel consumption of the engine and therefore of the vehicle deteriorates with the switchover into the homogeneous operation in an operating region of the engine in which no homogeneous operation would normally be necessary.
In a GDI engine, its consumption advantages are demonstrated mostly only in the stratified-charge mode of operation. For this reason, efforts are being made to cover as many operating regions with the stratified-charge mode of operation and to operate the internal combustion engines as long as possible in this operating mode.
The significance of GDI engines is that the engine is operated as unthrottled as possible (throttle flap open, intake manifold pressure corresponds to ambient pressure) independently of the power output or torque output. In this way, charge-cycle losses are minimized.